The 'tail' command skips the header. The perl command puts each single digit on its own line.

Visualizing this data set with coastlines overplotted is tricky. First, the grid is defined beyond the horizon of the Earth's disk, and GMT doesn't handle that well. So, the following shell script makes a subset of the grid using the GMT program 'grdcut'. See the comments in the script for details on that issue. If you want
to plot the whole disk with coastlines, try trimming the grid so that the disk is tangent with the edge of the grid, then plot the coastlines with -R-180/180/$minlat/90, where $minlat is the latitude where you trimmed the disk to.